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I know that a video interlace mode was available on the Apple /// prototype and eventually on the Apple ///+, but I haven't seen anything that says what it was for. Did it provide a higher resolution mode or was it simply to fill in the horizontal raster gaps?

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According to the Apple III Plus Owner's Guide, the interlace mode is called "text interlace":

text interlace a Feature of the Apple III console that, when activated, increases the resolution of text characters on the display screen.

Page 39 provides a little more detail:

The Apple III has text interlace, a function that allows you to turn on a second picture, or matrix of dots, which is in computer memory. When you turn on text interlace, the two identical sets of dots merge, creating a grid of 384 lines of 560 dots per line. The two sets of dots that make up the character *A* merge together, or interlace, to form a more crisply defined character.

So "text interlace" is simply a mode designed to enhance the appearance of text characters on the screen, without using any additional video memory, or even character ROM! It achieves this by switching from a progressive (non-interlaced) resolution of 560×192 pixels to an interlaced resolution of 560×384 pixels and doubling each row of pixels to stretch each 7×8 character into a 7×16 matrix. But note:

Text interlace works best with green-phosphor and other long-persistence video displays. If you have a black-and-white or color display, text interlace may make the picture flicker.

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